The present invention relates to a printed wiring device, and more specifically, relates to a printed wiring or circuit device with a simple structure, wherein a high-speed digital circuit or a circuit which handles high speed signals can be mounted.
Conventionally, as a printed wiring or circuit device, a multilayer circuit board has been used in the field wherein densification or integration is required. FIG. 2(a) is a sectional view showing a structure of a conventional multilayer circuit board. The circuit board is structured such that five insulation layers 20 through 24 are laminated, and on both surfaces of the circuit board and between the respective insulation layers, a grounding layer 25, a power supply layer 26, and four signal line layers 27 through 30 are provided.
It has been employed that the grounding layer 25 and the power supply layer 26 have solid and plane patterns in order to lower impedance as well as to make the signal line function as a microstrip line, and further to achieve a shielding function between the layers. Then, a width of the signal line has been determined so that characteristic impedance as the microstrip line is matching with output impedance of a mounted circuit.
In recent years, it has been requested that the electronic device is miniaturized and has a lighter weight, and in accordance therewith, the circuit board with a large number of layers is necessary. However, in the conventional printed circuit board as described above, in order to increase the number of layers with the same thickness, it is necessary to reduce the thickness of the insulation layers.
The characteristic impedance of the signal line has a function of w/h, wherein the thickness of the insulation layer is h and the width of the signal line is w (referring to Takeshi Yoshida, "Revised High Frequency Design Know-How", printed by CQ Press on Jul. 1, 1997, pages 67-80). Therefore, in order to obtain the same characteristic impedance by using the same material as the insulation layer and halving the thickness h of the insulation layer, it is necessary to halve the width w of the signal line.
However, there have been the following problems. As the signal line is narrower, the inductance constituent in the signal line is increased, so that the high frequency constituent in the signal is decreased to cause the signal delay or deformation. Also, when the width of the signal line is reduced, the device can not be manufactured by the existing manufacturing equipment, resulting in increasing the manufacturing cost.
An object of the present invention is to solve the aforementioned problems, and to provide a printed wiring or circuit device with a simple structure, wherein a high-speed digital circuit or a circuit which handles high speed signals can be mounted.
Further objects and advantages of the invention will be apparent from the following description of the invention.